Coumarin Schiff base derivatives: Synthesis, mesomorphic properties, photophysical properties and DFT studies

Coumarin Schiff base derivatives CS-(6-18) with alkyloxy chain on coumarin part and SCS-(6-18) with alkyloxy chain on both end of the molecule were designed, synthesized and characterized using different analytical techniques such as FT-IR, 1H-NMR, 13C-NMR and ESI-MS analysis. Compounds from the both homologues series were studied for their mesomorphic properties using differential scanning calorimetry (DSC) for the phase transition temperature, while the type of mesophase texture were studied using polarized optical microscopy (POM). On keeping alkoxy chain length fixed on coumarin end and varying different polar substituents on aldehyde end, only bromo substituted compound 12-CS-Br showed SmA mesophase, while compounds 12-CS-NO2, 12-CS-Nap and 12-CS-OV did not show any mesomorphic properties in heating and cooling cycles. In homologues series with variation of alkoxy chain on coumarin part in CS-(6-18), compounds of chain lengths 10-18 exhibited monotropic smectic A (SmA) phase only in cooling cycle. While on varying alkoxy chain length at both terminal end of the molecules in series SCS-(6-18), compounds showed an enantitropic stable smectic C (SmC) phase. All the compounds showed aggregation induced emission (AIE) in solid state under UV light (365 nm), however emission intensity were found to be dropped in solution. All the new coumarin Schiff base derivatives were studied using DFT theoretical calculations for their structural parameters & mesomorphic behaviour and compared with reported compounds in the literature.